Materials extracted from hops during a conventional brewing process include certain compounds referred to in the art as “α-acids”, having STRUCTURE I, and “β-acids”, having STRUCTURE II (and wherein R represents various, simple hydrocarbon groups, especially including isopropyl, isobutyl and sec-butyl), in combination with numerous, non-acidic organic compounds including fats, waxes, uncharacterized resins and steam volatile essential oils (especially certain mono- and sesqui-terpenes and oxygenated derivatives thereof).

During the wort boil, α-acids isomerize to compounds referred to in the art as “iso-α-acids”, having STRUCTURE III.
Unfortunately, as has long been recognized, malt beverages (i.e. beers including lagers, ales and stouts) brewed with normal hop products can develop “light-struck” flavors on exposure to light. These off-flavors are thought to be primarily caused by the photolytic action of near-UV wavelengths of light splitting off part of the isohexenoyl side chain of STRUCTURE III. The resulting dissociated, 1,1-dimethylallyl radical then reacts with naturally present compounds containing sulphydryl (—SH) groups to form a highly pungent mercaptan, 3-methyl-2-butene-1-thiol (MBT), which results in a sulphury aroma that is commonly described as “skunky” and generally considered to be unpleasant.
The prior art teaches processes wherein hop cones are extracted, and then that extract is fractionated to separate the α-acids, β-acids, and hop oils. For example, U.S. Pat. No. 5,917,093, incorporated herein by reference, teaches such a separation scheme. After separation, the isolated α-acids are isomerized to form the afore-described, light-unstable iso-α-acids which are known to impart the major portion of the traditional “bitter” taste to beer.
However, the prior art further teaches methods for the conversion of α-acids, iso-α-acids or β-acids to produce either tetrahydroiso-α-acids (“THIAA's”), having STRUCTURE IV, or hexahydroiso-α-acids (“HHIAA's”), having STRUCTURE V, which reportedly have much improved light stability.

These reduced forms of iso-α-acids are resistant to the above-described photolytic action and hence beers brewed with these types of hydrogenated iso-α-acids materials do not develop MBT derived off-flavors. U.S. Pat. No. 5,013,571 teaches methods for making THIAA's and HHIAA's. The current commercial practice is for these compounds to be marketed as mildly alkaline, aqueous solutions of their potassium salts in the same way as is also done for iso-α-acids. Their solubility is rather more limited, though. Hence, for example, THIAA's are normally sold at a strength of 10% w/w, while iso-α-acids are commonly offered as stable solutions at a strength of 30% w/w. Furthermore, because these compounds are substantially less soluble than are iso-α-acids, it is normal practice only to add them directly to fermented beer rather than to the unfermented wort (whether before, during or after the normal kettle boil) thereby avoiding an otherwise inevitable and substantial loss of bittering substances through precipitation. Clearly, this is an inconvenience since it is necessary to install and operate specialized dosing equipment in order to make the additions. Furthermore, it will be apparent that the subsequent beer will be lacking in the traditional “hoppy” notes deriving from the residual hop oils that would normally be present as a consequence of the addition of hops, hop pellets or hop extracts to the wort before the end of the boil.
The prior art also teaches that a light-stable beer can be produced from so-called rho-iso-α-acids (STRUCTURE VI), dihydrogenated derivatives that have been prepared by chemical reduction of iso-α-acids using sodium borohydride. U.S. Pat. No. 3,044,879 describes an early, commercially workable process to achieve this transformation.

Rho-iso-α-acids are more soluble than are THIAA's or HHIAA's and are usually sold commercially at a concentration of 35% (w/w) as measured by non-specific spectrophotometric (“Spectro”) analysis (but are usually found to have a true strength typically in the range 23-30%, as determined by HPLC). However, such solutions often precipitate during storage, necessitating inconvenient heating to re-dissolve the precipitate before the product can be used as a post-fermentation additive.
Some brewers consider that the β-acids have no particular value in brewing. For example, U.S. Pat. No. 4,918,240 teaches, inter alia, “the hop β-acids have generally been considered a useless constituent of the hops.” (Col. 2/1. 42-43). The '240 patent teaches a method for the removal of catalyst poisons useful in the conversion of the “useless” β-acids into desired THIAA's. Other brewers, however, consider that the total removal of the β-acids and essential oils from the wort in the normal brewing process is detrimental to achieving the desired flavor. Thus, for these brewers the production of a beer having improved light-stability and a flavor that is comparable with that of a conventionally brewed beer is only possible if the β-acids and hop oils are present in the wort kettle.
See also U.S. Pat. Nos. 3,798,332, 4,324,810 and 5,583,262 and European Patent Application 94301014.0.